bug#13409: [patch] make some error messages clearer

[Jim Meyering]

Benno Schulenberg wrote:
> On Fri, Jan 11, 2013, at 8:39, Jim Meyering wrote:
>>
>>       wwarn (_("%s: read failed"), src_name);
>>
>
> When things go wrong, I would prefer to see a word like
> "failed", "error", "mistake", "bad", "invalid" or "mayday"
> at the beginning of the line (right after the command name).
>
> cmd: error in something: /some/complicated/filename
>
> cmd: /some/complicated/filename: error in something
>
> The first form is to me much clearer than the second.
> That something went wrong is the main thing, with
> which file exactly is secondary, in my opinion.

Good argument, as long as there isn't a line number.
Also, you might argue that the active-voiced
  (I, the command) "failed to read"
is better than the passive-voiced
  (a) "read failed"

If there are both file name and line number, then they should be
formatted like this, like compiler diagnostics:

   CMD: FILE_NAME:LINE_NUMBER: diagnostic

so that diagnostic-parsing tools can handle them seamlessly.